Nuclear radiation detector



pri Z4, 1951 P. c. SMITH ETAL NUCLEAR RADIATION QETECTR Filed March 30,1949 Apriim, 1951 RQSMWH ETL 'f 2,550,610

NUCLEAR RADIATION DETECTOR Filed March 50, 1949 Sheets-sheet .2

@cf/fia? April 24, 1951 Filed March so, 1949 P. C. SMITH ET AL. NUCLEARRADIATION DETECTOR s 'sheets-sheet 5 Patented Apr. 24, 1951 NUCLEARRADIATION DETECTOR Perry C. Smith, Moorestown, and John H. Reisner,Haddonfield, N. J., assignors to Radio Corporation of America, acorporation of Delaware Application March so, 1949, serial No. 84,314

This invention relates to the detection of energy and more specificallyit relates to an vimproved form of detector for nuclear radiation orlike energy in which the eiect of unwanted radiations is rejected.

Radiation detectors have been utilized in the prior art which measurednuclear energy through the medium of phosphor screen detection andphotocell pickup means. These devices however are not entirelysatisfactory in that lextraneous energy also causes response of thedetection surfaces and therefore causes a small signal to noise ratio.

Our invention therefore contemplates novel means whereby unwantedradiations are rejected in a greater proportion than the wantedradiations, thereby giving an improved signal to noise ratio.

Accordingly it is an object of our invention to provide novel filtermeans for extraneous energy.

Another object is to vprovide a means for directly actuating a photocellwith scintillations Without such scintillations having to pass through aphosphor surface.

Another object of our invention is to provide an improved combinationwhereby energy in a narrow energy spectrum may be detected.

A still further object of our invention is to provide an improved formof` the scintillating screen type of detector for nuclear energy or likeradiation, which may be operated under conditions of high ambient light.

In order to illustrate our invention, a specific embodiment is describedhereafter, wherein particle radiations are allowed to impinge upon aphosphor Screener vscintillating crystal from which the resultingscintillations are detected by a, photocell. This embodimentillustratesfour invention in excluding extraneous light energy from thephosphor screen and the photocell by the use of novel filtering means.It is to be recognized however that this is only one specicembodiment ofour invention and that the invention is not limited to the particularenergy spectrum used here for purposes of illustration.

' The features of our invention which We consider novel are set forthwith particularity in the appended claims. The invention itself,however, both as to its organization and its method of operation,together with additional objects and advantages, will best be understoodfrom the following description of a specific embodiment, when read inconnection with the accompanying drawings wherein like referencecharacters'designate invwhich: v

5 claims. (c1. 25o- 71) `Fig. 1A shows the manner in which particle ra'-diation actuates a scintillation type energy detector of the prior art,

Fig. 2 indicates the effect of extraneous energyv Figs. '7 and 8 picturetwo assembled detectors 't which illustrate the manner of combining features of my invention according to the desired operatingcharacteristics.

Referring more specically to Fig. l, there is shown a surface havingVradio active material thereupon which is emanating radiation of say,such radiation i alpha particles. Hereinafter coming from nuclear orlike reactions shall be called particles. These particles are thendetected by means of aphosphor screen l, exposed,

within the range of the particles which range is represented by the arc6, drawn from the particle A. Between this screenand the radiationsource a light opaque screen 2, transparent to the particles isinterposed. A photo electric cell 3 is used as detector forscintillations caused when the particles'impinge upon the phosphor`surface, and a lens 4 focuses the scintillations upon the photocell. Alight tight housing 5 envelops the photocell and is connected to ,theopaque screen 2.

As to operation of the detector in Fig. 1, particles striking `asensitive phosphor l, e. g. ZnS with Cu activator, cause` the emissionof light which is focused by the lens 4 on the photocell 3. l

scintillations activate the photocell which passes n on intelligence toa suitable audible or visual indicator through appropriate electricalmeans.

It is to be noted that the height of the phosphor above the surfacebeing examined determines the.

angular width in whichthe particles canbe de-V tected. A measure o fthis width is the angle shown. To fully understand this discussion, it--is to be realizedthat particles generally travel al well defineddistance radially from a source with no directional properties. In orderto prevent the photocell from being made operativeA by light radiationslpassingthrough the scintillating screen, a thin light opaque Vcoat maybe evaporated upon the phosphor. This coat must be light opaque and yetthin enough to present a minimum barrier or stopping agent for theparticles with respect to the scintillating screen.

Since impact with matter is the means of stopping such-particleaiit isnecessary tomakey screens of materials composed of elements-or lowatomic number, e. g., from lithium to aluminum.

In Fig. 2, the manner in which light enters the window of the detectorassembly isshovvn. .,One means is direct reilection, the other'diffusescattering. The latter is more importantfiand it 'may be determined thatthis lightfenteringthe Window area is proportional to thefsolidr anglep.'It i is true that qs is greater than theangle subtended by thephotocell and the Window; however, light scattering from the phosphorcauses a portioniof all light in the solid angler to be scattered intothek angle Aof the photccell In practice it is difcult'tofproduce athin-light opaque barrier free of pinholes admitting part of this-light.Iffthe barrier is made; thick enough to insure light opaolueness,.itgreatly reduces the effective distance from the source vat` Whichtheparticles may be detected.

The ratio ofA ,ambient light intensity in` a. room to the minimumscintillation light intensity from the radiation detecting element maybe roughly estimatediaslOl. .When using Idetectors having particularspectral.A response` characteristics. the bestf spectral response curvecannot be .expected togcha-ngethe; ratio morethan two orders. Obviously`an opaquescreen could befused to exclude the 105 times phosphorlight-intensityv available where-the instruments' araused. Howevenso farit.- hasbeen impossible tolmake .sufficientlythinscreens;Withoutpinholeswhich still leakminute portions-.lof the outsideilluminationonto the photocell.

. 'It is one j'purpose-of this-invention 1 to` provideassociated=meansvforrthe -reductionrof ffleakage light to a negligibleproportion so'that scintillation type meters.mayV be used .inhigh-ambient illumination.

\.When -.inoperation, the scintillation survey metertgeneraily operates`closeto asurfacafe. g., notganore ,than 'two .inches away. Illuminationstriking thesensitiveportions ofsuch a survey meter must -rlrstcomebyreection or scattering Y from thesurface beinglsurveyedas shown inFig. 2. :Since incident .illumination isla-rgely diffuse, the intensityof the spurious light is ,proportional tothe :solidY anglesubtendedby.the .opening window of the device. anglefor straylight more rapidlythanthe solid angle for. activating particles,A itispossible to improvethe ,signalto stray lightratio.

AFig. 34 therefore showsthe samearrangement as Fig. .3 .with .theadditionoidirecting varies, 1. All intense light man`` angle greaterthan e is absorbed4 in the vanes .which ,have nonrelecting internalsurfaces. The ratio offp and fp.. as shown is aboutrf5. .The vanes.aslchosenhave the same value of as the case where no varies are used.There is, however, a portion of the incident particles stopped by thevanes. See path B. This portion is never more than one half. The ratio,therefore, ofjparticles toy stray iight/has been irnproved by a'factorof2.5,- by the use'or" such vanes. VAProportions of the vcellular vanes,1, determine the signal to `noise Aimprovement by changing the ratiov ofangles-and e andA this ratio may be chosen -to suit theparticularrequirements off such a-'detecton is-a function 0f the Byi :decreasingthe solid 4opaque screens. .natesthe phosphor from an angle 4 butscatters 'internally only in the angle fp representing a grdecreaseintotal lightof a factor of about twenty. l"All impactsby .particles arecounted but less of -theemitted light reaches the photocell. Theresulting improvement in signal to noise ratio is dimensions w and eshown in the diagram in the order,

da =2 tan- 12g Where the cell cross section is circular. The ratiop/41as shown is about 5.

`In Fig. 41 thesame angular set-up is shown as in Fig. 3, but the vanesfollow the phosphor and Here the incident light illumiweer@ wistheindieated width ofthe-cen ancre is the length -or depthgoi the-cellin thedirection substantiallyparallel tothe direction ofthe radiationbeing measured.

If the non-reiiecting substance used as a coating Qn the internal vanesis modied las in Figi 5 y to anon-reflecting phosphorcoatingj.Vafp-further improvement of the'signal to noise ra-tiowill; result asfollows. The opaque screen'Zlstops most of Vthe light except that,entering pinholes, lbut the phosphor absorbsmuch or theilight enteringthese pinholes. vWhen b is made 35 the activating particles entering thedetector at ,less than 17.5 do vnot strike the-phosphor. vAll particlesentering at greater angles than 17-.5ov but less than 37 strike thephosphor obliquely Wcausing the emission of light. This represents aloss of only 20%` of the particles, but a much greater loss in thescattering from incident light. There is also a 'loss from the,lightemitted by the phosphor, but again this is in a smallerproportionthan the less for scattered stray light.

It is to be noted that lightA from the scintillations does not have topass through the phosphor surface but is directly transmitted to thephotocell in-this particular embodiment, therebyigiving a larger signaloutput for anygiven particle collision.

The geometry shown in Fig.;5 was chosenas it represents the poorestratio of signal to noise. When 4the spacing between probel andlsurfaceis y decreased below thatshown; the incidence of theVparticlesincreases. The angle 0/2.=;53-can .be seen to be the smallestangleaat which a .particle emitted from a surface with a '7Acmfrange'can strike ascreen 5.4 cm. awayfrom the surface.

,Then solid angle, AWithin whichl emission'of aparticle will give riseto scintillation-will be measured from the normal to the screen oremitting surface and is the complement of /2 or 37C as shown.

Since the particles are non-directional, we'may expecta numberofvpartcles inria solid angle -p-roportional to the size of that solidangle. -In=-l"ig.

5, is 35, which effectively describes a cone whose elements are inclined17.5 to the altitude of the cone. Such a cone forms a solid angle of0.08 1r radians at its vertex. 'I'he solid angle formed by the effectivecone, whose base angle is 53, is 0.40 1r radians. Only those particlesfalling in the angles between the two cones are registered. The solidangle between the two effective cones is 0.32 1r radians. Thus, 0.08 1rradians out of 0.40 1r radians are useless, or 20% of the particles arenot counted. The loss of registrations caused by stray light however isobviously increased in a much greater proportion because only the lightincident in the 0.32 1r radians is registered excluding that lightbetween qa and 0.32 1r radians or approximately 75 As shownhereinbefore, qs is the important quantity in design. Where qi is 35 asin Fig. 5, the ratio w/e is 0.32. This is the condition where w=2.5 mm.and e=7.5 mm. c depends primarily upon the width of the sensitive windowof the probe and of the length of the lip extending beyond the surface2. The width of the probe window shown in Fig. 4 is 0.75 inch while thelip is 0.125 inch. This gives a =l60 as shown.

Fig. 6 shows the effect of employing spectral emission, response andenergy characteristics. When the use of a photo surface having a narrowspectral response, e. g., RCA S-4, and a corresponding phosphor surface,e. g., RCA#5, the response at 3500 will be maximum. An optical lightfilter, 9 in Figs. 7 and 8, also peaked at 3500 may be used. Under suchconditions the signal to noise ratio may be improved several fold insunlight and to a great degree in tungsten lighted areas. It is to berecognized however these particular surfaces and values are merely usedto illustrate the means of eliminating unwanted energy, and that theinvention is not to be restricted to such values.

Complete assemblies are shown in Figs. 7 and 8, using vanes to improvethe signal to noise ratio and lters to pass the strong energy band tothe exclusion of stray energies. These are only two distinctcombinations and it is obvious that in the light of our disclosure thatimproved results may be realized by different combinations such as usingvanes both preceding and following the phosphor.

It should be pointed out that the calculations contained herein are notexact but are estimates frequently not taking into account 'complicatedgeometry which might bring about minor modication of the estimatedvalues.

Our invention shows a novel vane system used as an energy lter incombination with components having particular spectral energy response,however, it is not intended to be limited to the illustratedembodiments, and there may be suggested by our disclosure to thoseskilled in the art certain modifications which will not necessarilyconstitute a departure from our invention.

Having thus fully described the nature, construction and operation ofour invention, we Wish to secure by Letters Patent and claim:

l. A radiation detector comprising in combination, a substanceresponsive to said radiation, detection means for said response, afilter which conducts said radiation interposed between said radiationand said responsive substance, and directive vanes substantiallyparallel to said radiation having said substance as a surface coatingexposed to said radiation and being located between said radiation andsaid detector.

2. In combination, a radiation source, a substance responsive to saidradiation having a substantially non-reflecting surface, cellular vanesmounted substantially parallel to said radiation coated with saidsubstance, and means for detecting said response.

3. A radiation detector comprising in combination, cellular vanessubstantially parallel to said radiation having dimensions whereby wideangle light energy is rejected, surface coating on said vanes ofsubstantially non-reflecting substance responsive to said radiation,detection means for said response, filter means interposed between saidvanes and said radiation and light opaque housing means connecting saidfilter and said detector.

4. The combination as recited in claim 2, having an optical light filtermeans interposed between said radiation and said detecting means.

5. The combination as recited in claim 2, having an optical lter meansinterposed between said vanes and said detecting means.

PERRY C. SMITH. JOHN H. REISNER. REFERENCES CITED The followingreferences are of record in the le of this patent:

UNITED STATES PATENTS OTHER REFERENCES A New Precision X-RaySpectrometer, by W. Soller, Physical Review, vol. 24, 1924, pp. 158-167.

1. A RADIATION DETECTOR COMPRISING IN COMBINATION, A SUBSTANCERESPONSIVE TO SAID RADITION, DETECTION MEANS FOR SAID RESPONSE, A FILTERWHICH CONDUCTS SAID RADIATION INTERPOSED BETWEEN SAID RADIATION AND SAIDRESPONSIVE SUBSTANCE, AND DIRECTIVE VANES SUBSTANTIALLY PARALLEL TO SAIDRADIATION HAVING SAID SUBSTANCE AS A SURFACE COATING EXPOSED TO SAIDRADIATION AND BEING LOCATED BETWEEN SAID RADIATION AND SAID DETECTOR.